1. Technical Field of the Invention
The present invention relates to a line driver having a self adjustable output impedance and, in particular, to a transformer line driver with a hybrid.
2. Description of Related Art
Line drivers having a controlled output impedance are well known in the art. See, B. Nauta, et al., xe2x80x9cAnalog Video Line Driver with Adaptive Impedance Matching,xe2x80x9d ISSCC98, pp. 318-19, 1998. A simplified schematic of one such driver 10 is illustrated in FIG. 1A. The driver 10 (also referred to as a xe2x80x9cbufferxe2x80x9d) comprises an operational amplifier 12 whose negative input terminal receives an input voltage Vin. The output terminal of the operational amplifier 12 is connected to the gates of two field effect transistors 14 and 16, where the illustrated xe2x80x9cNxe2x80x9d value is equal to the ratio of their respective drain currents. The sources of the field effect transistors 14 and 16 are connected to a reference voltage Vdd. The drains of the field effect transistors 14 and 16 are connected to each other by a resistor (R1) 18. The drain of the field effect transistor 14 is connected in a feedback fashion to the positive input terminal of the operational amplifier 12, and is also connected to ground through a resistor (R2) 20. An output voltage Vout is supplied from the drain of the field effect transistor 16 to drive a transmission line 22 having a characteristic resistance equal to the load resistance (RL) 24. By properly selecting the values of the resistors R1 and R2 for the driver 10 in a well known manner (and as illustrated) with respect to the xe2x80x9cNxe2x80x9d value and the value of the load resistance RL, the value of the output impedance from the driver may be set (i.e., controlled) substantially equal to the load resistance RL. An advantage of this driver is its reduced power dissipation which makes it very attractive for implementation in an integrated circuit. However, with respect to an integrated circuit fabrication, the precise resistance values needed to achieve substantial matching of driver-line impedance are very difficult to consistently obtain.
It is recognized that it would be advantageous to be able to exercise some adjustment control over the output impedance of the driver following the setting of the resistance values. The driver of FIG. 1A may be modified, as shown in FIG. 1B, to provide for such an adjustment mechanism. Controllable source degeneration (through circuit 30) is applied to the transistors 14 and 16. The current ratio value xe2x80x9cNxe2x80x9d is electrically tunable (through circuit 30) via application of the voltage Vtune. In this implementation, the driver adapts to match the load resistance RL using a control loop 28 that integrates the current from the output of the transconductance amplifier (28), which results from the voltage output of the drain of transistor (16) from the transistor 16 to generate Vtune for application to circuit 30 resulting in an adjustment to the source current of transistor 14 and a change in the value of N. At low frequencies, the control loop 28 forces Vout to equal Vin, in which case the gain of the driver is one. By then setting the resistances R1 and R2 as discussed above, approximate matching of the output impedance to the load resistance RL is obtained, with the control loop 28 further refining the matching.
Most telecommunications devices utilize transformer decoupling of the driver and the transmission line. Because transformer driver-line decoupling is typically utilized in the push-pull configuration, a direct current output signal related to the load resistance is not available to be integrated by the control loop 28 and produce the adjustment signal Vtune. Furthermore, if the transmission line is relatively long, its direct current resistance is substantially different from the characteristic impedance. In such situations, the precision of the impedance adjustment provided by the FIG. 1B circuit is not sufficient.
One solution to this problem is presented in R. Mahadevan, et al., xe2x80x9cA Differential 160 MHz Self-Terminating Adaptive CMOS Line Driver,xe2x80x9d ISSCC2000, pp. 436-37, 2000, where the gain of the transformer push-pull driver is adjusted to unity by using the low frequency content of the transmitted signal. In this implementation, the driver output signal is filtered and compared with the input signal. Responsive to that comparison, the driver transistor ratio is adjusted to set the gain to unity. This method of driver gain adjustment is effective if the load of the driver is a transmission line having a matched termination at the opposite end. However, in a full duplex architecture where transmission and reception occur through the same line, a similar driver should be located at the opposite end of the transmission line. Typically, at the beginning of the adjustment procedure neither one of these drivers is matched to the line. This causes significant reflections on the signals, which affect the amplitude of the signal at the driver output, and the simultaneous adjustment of both drivers becomes a complex multi-step routine.
In some applications, transmission and reception take place simultaneously through the same transmission line. A hybrid device or circuit is typically connected to split the transmitted and received signals. It is conventional to utilize voltage mode drivers in modern wireline communications devices. In such cases, additional resistors are often connected in series with the line driver to effectuate line impedance matching. As an example, these additional resistors may be used to build a balanced bridge hybrid circuit. Unfortunately, the differential output of such a circuit has a common mode voltage equal to the transmitted signal, and this results in a substantial increase in transmitted signal echo. As a further drawback, if such a hybrid circuit is used in a self-terminated driver there is a substantial reduction in power saving efficiency.
There is accordingly a need for a line driver possessing a self-tuned output impedance and operable in an efficient manner with a hybrid for application in communications devices where transmission and reception occur simultaneously over the same transmission line. Such a driver would preferably be inexpensive to fabricate and present a relatively simple method for tuning gain, adjusting output impedance and balancing the hybrid.
A line driver circuit with hybrid is provided for connection to a signal transmission line. The circuit includes a controlled or synthesized impedance buffer. The line driver circuit further includes an adjustment circuit that processes an output from the hybrid during training mode and generates an adjustment signal for application to an adjustable controlled current source within the buffer. By manipulating the adjustable controlled current source with the adjustment signal, the output impedance of the buffer can be made to substantially match the characteristic impedance of a transmission line connected to the driver.